1. Field of the Invention
The present invention relates to a construction of a piston compressor by which an operating noise thereof is reduced.
2. Description of the Related Art
Known in the prior art is a piston compressor of a type provided with cylinder blocks having cylinder bores, inlet and outlet ports, pistons reciprocally and slidable arranged in the respective cylinder bores, compression chambers formed on sides of the respective pistons, valve port plates forming inlet and outlet ports, and inlet and outlet valves made of thin metal web for controlling the admission of gas via the inlet ports into the compression chambers and a discharge of the gas via the outlet ports from the compression chambers, respectively.
In this type of the compressor, the pistons are connected to a means such as a swash plate, to obtain a reciprocating movement of each of the pistons between axially spaced first and second positions (dead centers positions), and as a result, during one complete rotation of the swash plate, the movement of the piston in one direction toward first dead center position at which the volume of the compression chamber is increased allows the gas to be introduced into the compression chambers via the inlet valves, and the movement of the piston in the opposite direction toward the second dead center position at which the volume of the compression chamber is reduced allows the gas to be discharged from the compression chambers via the outlet valves. In this type of compressor, the piston must be located as near as possible to the valve port plates to obtain as small as possible a clearance between the piston and the valve plates when the piston is at the second dead center position, while preventing the piston from coming into contact with valve port plates within a permissible tolerance of the clearance between the pistons and the valve plates and the valve port plates when assembled, whereby an increased compression efficiency is obtained.
However, the increase in the compression efficiency by reducing the clearance between the pistons and the valve plates to a permissible limit, even if within a range of tolerance, can generate an "over compression" phenomenon whereby the pressure in the compression chamber becomes much higher the discharged gas pressure at the outlet chambers, which subjects the peripheral units to an excessive pressure and often causes damage thereto, and further, causes a generation of noise because the outlet valve is brought into violent contact with the retainer plates thereof. Such an over compression is due to the existence of the lubricant mist in the gas to be compressed. Namely, when a thin metal web outlet valve is at a position at which it closes the outlet port, lubricant held between the outlet valves and valve port plate generates an adhesive force therebetween, and as a result, the outlet valves are adhered to the valve port plate. In the prior art, this force becomes relatively large because the valve port plate has a very smooth surface on the order of 1.6 to 3.2 .mu.mRz, needed to maintain a desired sealing between the outlet valves to the valve port plates and to obtain a desired sealing of the compression chambers. The large adhesive force between the outlet valve and the valve port plate causes difficulties in separating the outlet valve from the valve port plate, causing the pressure in the compression chambers to become much higher than the pressure at the outlet port, whereby an over compression occurs.